Polymers in various formulations have been used for decades as insulation for conductive wires. A commonly used polymer is polypropylene. Polypropylene is readily combinable with many other olefin polymers, and is manufactured in several different grades and in formulations with numerous other polymers by many manufacturers.
Existing wire and cable grades of polypropylene suffer, however, from several problems when the polypropylene is deposited at high line speeds. Probably the most significant of these problems is unstable flow pattern that occurs at the exit point where the polymer is extruded from an extruder die. Instability of the flow pattern results in an insulation wall that surrounds the conductor with varying centering properties. Further, the diameter of the insulation is nonuniform as a result of the unstable flow pattern. Uniformity in both of these two parameters is critically important, particularly for data carrying products such as LAN cables and associated wiring, most significantly the T1 cables and category 5 and higher cables.
Another problem associated with wire and cable grades of polypropylene deposited at high line speeds is an often-arising difficulty in achieving an acceptable bond to the conductor. Without an adequate conductor-insulation bond, poor impedance stability results in data cables. Consequently, high scrap losses result at the production stage.
Polyethylene is another common insulation material. There are difficulties associated with deposition of polyethylene insulation as well. For example, existing flame retarded high density polyethylene tends to generate excessive extrusion pressures when deposited at high line speeds, which consequently limits production rates. Also, it is common to see the extrusion tooling, including the screw that powers the extrusion, with an acquired coating or plate out of the flame retardant chemical during production. After some time, this plate out flakes off and causes numerous electrical faults in the coated wire. This nuisance suspends production until the affected parts are cleaned.
The present invention is directed to overcoming the problems of unstable flow and variable bonding between a conductor and insulation in a cable or wire at high line speeds which are typically about 6,000 ft/min (about 1828 meters/min) but can surpass 7,000 ft/min (about 2133 meters/min). The present invention is also directed to overcoming the problems of high extrusion pressures in thin wall extrusion, while concurrently eliminating the plate out problem. These improvements result in enhanced production rates and reduced scrap losses.
The invention is firstly directed to a method for manufacturing a conductor. First, at least one conductive wire is provided. Next, an insulating material is extruded around the conductive wire. The insulating material should be extruded directly onto and around the conductive wire. The extruding step can be performed at a speed that is greater than about 2,100 meters of conductive wire per minute.
The present invention is also directed to a conductor that is made according to the above method and includes at least the conductive wire and the insulating material. The insulating material includes at least a first medium impact grade polypropylene copolymer, and another polyolefin. The polypropylene is generally isotactic in structure. The first and second medium impact grade polypropylene copolymers each have a density of approximately 0.9 g/cm3. The polyolefin is preferably a very low density polyethylene (less than or equal to 0.906 g/cm3).
The extruding step includes extruding the insulating material at a thickness that is no greater than about 0.25 mm. The conductor is normally approximately 0.52 mm in diameter.
The insulation most preferably further includes a second medium impact grade polypropylene copolymer, where the first medium impact grade polypropylene copolymer has a melt flow index that is greater than the second medium impact grade polypropylene copolymer melt flow index. The second polypropylene also has a generally isotactic structure. The insulation material can also include at least one flame retardant. It is preferable that the flame retardant include at least one inorganic compound.